Perpendicular magnetic recording (PMR) has become the mainstream technology for disk drive applications beyond 200 Gbit/in2, replacing longitudinal magnetic recording (LMR) devices. Due to the continuing reduction of transducer size, high moment soft magnetic thin films with a Bs above 22 kG are required for write head applications. PMR uses a magnetic yoke surrounded by field coils that terminates in a single pole that is used for the write head. The write pole must be wide enough at one end to attach to the yoke and narrow enough at the other end to confine the write flux to a very small area typically about 0.1×0.1 microns.
A conventional PMR write head as depicted in FIG. 1 typically has a main pole layer 10 or write pole with a pole tip 10t at an air bearing surface (ABS) 5 and a flux return pole (opposing pole) 8 which is magnetically coupled to the write pole through a trailing shield 7. Magnetic flux in the write pole layer 10 is generated by coils 6 and passes through the pole tip into a magnetic recording media 4 and then back to the write head by entering the flux return pole 8. The write pole concentrates magnetic flux so that the magnetic field at the write pole tip 10t at the ABS is high enough to switch magnetizations in the recording media 4. A trailing shield is added to improve the field gradient in the down-track direction.
Referring to FIG. 2, a top view is shown of a typical main pole layer 10 that has a large, wide portion called a yoke 10m and a narrow rectangular portion 10p called a write pole that extends a neck height (NH) distance y from the ABS plane 5-5 to a plane 3-3 parallel to the ABS where the pole intersects the yoke at the neck 12. The main pole layer 10 flares outward at an angle θ from a dashed line 11 that is an extension of one of the long rectangular sides of the pole 10p. PMR technologies require the write pole 10p at the ABS to have a beveled shape (as viewed from the ABS) so that the skew related writing errors can be suppressed. In other words, the top edge 10a of the main pole layer 10 usually overhangs the lower edge 10b by a certain amount.
Although a PMR head which combines the features of a single pole writer and a soft magnetic underlayer has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density, PMR still faces some challenges. One major issue is related to trapezoidal write pole plating or the so-called through via plating in the semiconductor industry. In particular, there is a tendency to form void defects on the bottom and sidewalls of the write poles. Void defects are undesirable since they can lead to corrosion in the downstream slider process and adversely affect writer performance and wafer yields.
One cause of void defects is considered to be air bubbles trapped on the seed layer surface inside narrow openings when the wafer is submerged into a solution prior to plating. Poor wettability or hydrophobic characteristics of the seed layer make it difficult for small air bubbles to escape from a high aspect ratio cavity layer. In a conventional plating process, seed layer wettability is improved by the addition of excess surfactant such as sodium lauryl sulfate or sodium dodecyl sulfate to the plating solution. Unfortunately, surfactant may decompose during plating and can be co-deposited into the plated film. Higher surfactant loadings can easily raise the impurity level in the plated write pole and thereby lower its corrosion resistance and writability. Surfactant can also interact with other additives in the plating solution to form unwanted nodules in the plated film.
H. Gu et al. in U.S. Patent Application Publication 2007/0080067 provide a method for reducing the formation of void defects on the surface of a substrate during Cu plating by oxidation of the plating seed layer prior to substrate immersion. However, this method cannot be applied to high magnetic moment write pole plating where a Ru seed is generally used as a plating seed, write gap as well as a CMP stop layer. Oxidation of a Ru seed layer forms ruthenium oxides on the plating seed surface which improves the seed wettability in plating solution. However, the plated high magnetic moment materials such as CoFe adhere poorly to ruthenium oxides, resulting in plated film delamination. Furthermore, ruthenium oxides cannot be removed by immersion in an acidic plating solution typically used for write pole formation.
In U.S. Pat. No. 7,449,098, a method is disclosed whereby a metal is selectively plated into recessed regions. An additive such as an alkanesulfonic acid becomes selectively attached to the surface of recessed regions by selective removal from exposed regions using a mechanical rubbing process with a pad. The additive increases the rate of metal plating on recessed surfaces compared with exposed surfaces to minimize the amount of excess plated metal to be removed in a subsequent process.
U.S. Pat. No. 7,442,267 describes a method of annealing a Ru seed layer in an oxygen free atmosphere to reduce oxides and thereby reducing Ru resistivity before immersing the substrate in a plating solution. Related patent application Ser. No. 10/915,865 teaches a multi-step immersion process during plating to minimize bubble formation on electroplated surfaces.